1. Field of the Invention
The present invention relates to a scanning optical microscope and, more particularly, to a scanning optical microscope having a confocal optical system.
2. Description of the Related Art
A confocal optical system is an optical system for focusing light to the surface or inside of a sample through a lens to form a minute spot and converging reflected light or fluorescence from the minute spot by a detector, etc. through a minute aperture formed in conjugation with the spot. In this confocal optical system, only the light from a minute spot on a focal plane (hereinafter referred to as a spot) enters the detector through the aperture, but light from before and after the spot is not able to pass through the aperture, nor does it enter the detector.
A scanning optical microscope having this type of confocal optical system scans the minute spot and processes information detected by the detector in synchronization with position information of the minute spot to obtain an image of a spot scanning surface. The spot scanning surface can freely be set inside of an optically transparent sample, and an image of a section of the sample having a specific depth can be formed accordingly. This type of scanning optical microscope has resolution in the direction of an optical axis, and enables a so-called optical slice. The formed image has a good contrast and high resolution since the light from before and after the spot in the direction of the optical axis does not pass through the aperture.
In the confocal optical system, usually, a single convex lens is used as a means for converging the light from the spot. When the focal distance of the convex lens is short, the diameter of a beam waist is very small, i.e., several tens of micrometers. Since, in the confocal optical system, the diameter of an aperture has to correspond to that of the beam waist, if the diameter of the beam waist is very small, it is difficult to optically control the aperture. On the contrary, when the focal distance of the convex lens is long, the diameter of a beam waist is large and that of an aperture is also large. This aperture is therefore easy to optically control, but the confocal optical system is increased in size because of the long focal distance.
Furthermore, a scanning optical microscope is proposed in which a proper amount of light enters a detector through an aperture by virtue of a variable diameter of the aperture thereby to prevent a dark image due to a very small amount of light passing through the aperture from being formed. One example of this scanning optical microscope is disclosed in GB 2184321 B or WO 90/00755. The diameter of the aperture is varied by use of an iris.
The scanning optical microscope is very useful for setting a condition such as positioning of a live sample to be observed and comparison of a sectional image and a total image if a nonconfocal optical system can be used in addition to the confocal optical system. The nonconfocal optical system can be achieved by removing an aperture from the confocal optical system. However, optical control is required whenever the aperture is rearranged to constitute the confocal optical system. There is another possibility that the nonconfocal optical system can be achieved by using an iris and by changing the diameter of the iris. Since, however, the ratio of the maximum diameter of the iris to the minimum diameter thereof is required to be about 10, the nonconfocal optical system cannot be achieved in actuality.